Contactless confined readout of quartz crystal resonator sensors

A configuration and method for contactless readout of the resonance response of a thickness-shear mode quartz resonator sensor is presented. The configuration uses a crystal with a large common electrode on the front face, and one, or more, small electrodes on the back face, leading to localized sensing regions. Each back electrode is capacitively coupled to a separated tip electrode. The tip consists of a small disc and a guard ring, which confine the electric field to the electrode area and make the measurement unaffected by the stray parallel capacitances. Analysis shows that the proposed configuration can be modeled as an extension of the Butterworth-Van Dyke (BVD) circuit and that, by a proper choice of the reference frequency around resonance, it is possible to obtain frequency readings that do not depend on the tip-to-crystal stand-off distance. Experimental results show that a mass load added on the front electrode can be consistently detected and measured irrespective of the tip-to-crystal stand-off distance